The present invention relates generally to magnetic resonance (MR) imaging systems and methods. More particularly, the present invention relates to a MR imaging system equipped for real-time imaging and methods for assisting the operator to interactively prescribe the geometry of the excitation profile of a structure of interest for subsequent acquisition of a MR image of the structure of interest.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field Bo), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment M. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
When attempting to define the volume of coverage of an MR imaging scan, the NMR system operator may desire to quickly view a preview MR image (such as a real-time MR image) of the anatomical section within this volume of coverage. This process can be particularly useful when prescribing a three dimensional imaging volume, in which the desired high spatial resolution requires the thinnest slab possible. It is desirable to position this thin slab such that the anatomical section within the volume of coverage is complete, i.e. for example, covers the entire desired vascular network. Thus, a quick view of each side of the slab prior to initiating the three dimensional acquisition is useful for insuring that the entire anatomical section desired is within the defined volume of coverage.
Typically, two dimensional axial, sagittal and coronal xe2x80x9cscoutxe2x80x9d images are first acquired. Such scout images are stored for later use. To use, the operator calls up the scout image and either graphically or explicitly (using geometry coordinates) prescribes the imaging volume directly on the scout images. The imaging volume may be either a two dimensional stack of slices or a three dimensional slab of the structure of interest. The drawback of this technique is that the operator does not actually see the results of the prescribed geometry until the subsequent imaging volume is acquired. Prescription errors cannot be detected nor corrected until the imaging volume acquisition is complete. Thus, when prescription errors exist, the operator is required to re-prescribe and re-acquire the imaging volume of the desired anatomical section.
One embodiment of the invention relates to an MR imaging system having interactive MR geometry prescription control. The MR imaging system provides a method for prescribing geometry to a subsequent imaging volume of a structure of interest. The operator interactively acquires and displays a first real-time imaging section using an input device. Using the input device, the operator sets the first geometry information defining the scan plane corresponding to the first real-time imaging section in a buffer of the MR imaging system. The first geometry information prescribes the xe2x80x9cstartxe2x80x9d boundary geometry of the subsequent imaging volume. Next, the operator interactively acquires and displays a second real-time imaging section using the input device. Similarly, the operator sets the second geometry information defining the scan plane corresponding to the second real-time imaging volume in the buffer of the MR imaging system. The second geometry information prescribes the xe2x80x9cendxe2x80x9d boundary geometry of the subsequent imaging volume. Henceforth, the boundary geometry defining the desired subsequent imaging volume, contain within it the desired structure of interest such as a vascular network, can be efficiently and rapidly checked and prescribed prior to initiating acquisition of the desired imaging volume.
Another embodiment of the invention relates to retrieving geometry information of a previously prescribed imaging volume. Using the input device, the operator selects a previously prescribed imaging volume and the system loads the xe2x80x9cstartxe2x80x9d and xe2x80x9cendxe2x80x9d boundary geometry information corresponding to the selected previously prescribed imaging volume into the buffer. The operator can then acquire and display real-time imaging sections using the xe2x80x9cstartxe2x80x9d and xe2x80x9cendxe2x80x9d boundary geometry information retrieved from the previously prescribed imaging volume. The operator typically uses a graphical user interface in conjunction with the input device and a display screen for interactively prescribing the MR geometry of excitation profiles of structures of interest.
It is an object of the present invention to provide a feature which allows the operator to utilize the speed of real-time imaging section acquisition and display thereof to accurately and efficiently prescribe the desired imaging volume boundary geometry prior to committing to the imaging volume acquisition. Another object of the present invention is to allow the operator to retrieve boundary geometry previously prescribed for an imaging volume, to rapidly view imaging sections corresponding to the retrieved boundary geometry, and to modify the boundary geometry, if necessary, prior to committing to the image volume acquisition.
Other principal features and advantages of the present invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description and the appended claims.